One year ago: the eve of Snowpocalypse

By
Jason Samenow

Surface weather map on Friday morning, December 18, 2009. Low pressure, which started forming in the Gulf of Mexico Dec. 17, was beginning its journey toward the mid-Atlantic. Source: National Weather Service

Three hundred sixty-five days ago the hype was building and grocery store shelves were about to be under siege as the Washington's biggest December snow on record started to organize in the Gulf of Mexico.

Let's take a look back at that historic, crippling storm...

In the days leading up to the event, the overall pattern and model guidance suggested potential for a storm (see here and here), but not until 24-36 hours before the flakes began falling did it become clear this would be a biggie. We decided to sound the alarm bell Thursday evening, December 17 with the following message:

Current satellite imagery from the Gulf of Mexico shows this storm firing up. All of the latest data coming in continues to support the potential for 6" of snow or more this weekend as the storm moves northeastward up the coast. Totals exceeding a foot are possible in some locations.

In the end, 16-24+" was about how much fell. The 16.4" that fell at Reagan National was not only the largest December storm on record, but also more snow than had fallen in any previous December month on the books. It was the 6th biggest two-day snowstorm in any month.

After the storm, we posted a number of recaps about what happened and why, which are fun to review:

What I remember is that less than -24- hours before the snow started to fall on Friday night "CWG's latest forecast still calls for a 15% chance of less than an inch and only a 50/50 chance for more than 6"” - and that was with consistent model runs for days ahead of time and after NWS warnings of a looming historic event. CWG seems to be doing a better job of flagging the possibilities further in advance this year... we'll see!

On our annual trip to DC, we left on Friday from Indiana and at the last minute we threw our cross-country skis in the car. Little did we know that we would spend Sat and Sun skiing from our hotel in Arlington, over to the monuments, down the Mall! It was the best time ever! I was glad to have the museums reopen om Monday, but we will never forget the wonder at skiing on Sat when the blizzard was still going strong, and not seeing the Wasington Monument till we were almost under it!
We are on our way in the car right now, but brought our bikes, not the skis this time.

It is false that models were either consistent or in agreement days ahead of time in simulating a major, historic event. There was a lot of variability in the model with many (even most?) forecasters noting the significant possibility of a glancing blow type of storm track.

As a weekend manager at a downtown hotel I was constantly monitoring the weather reports, mostly online. During the day Fri. I decided that this storm was really real. I left the hotel in the afternoon to go home and pack some clothes for the weekend. Upon my return to the hotel I dropped off several days of clothing and some snacks and went to park my car. As I walked back to the hotel the first few flakes began falling around 9 PM and I congratulated myself for perfect timing. I'm glad I continued to monitor the situation on Sat., otherwise I wouldn't have been able to tell housekeeping and other departments that Metro was closing at 1PM. By 2 I think the only people left were a few guests, some restaurant workers, 1 front desk clerk and me.

Thank goodness some housemen made it in Mon. to receive a large shipment of furniture on a monster trailer that I didn't think would make it in from Minnesota. Finding a place to park it was quite an experience and then I sat at the front door freezing as items were checked in. The sun was brilliant, but I had to wear my outdoor clothing and gloves.

It was fun watching some of younger guests playing in the snow, some for the first time ever seeing the white stuff.

12 hour days, maybe a school group stuck at the hotel, plenty of food in the restaurant, but I can't remember which day I finally went home.

Jason - I don't want to spread false information - but your summary implies that on Thursday you were calling for 6" and maybe a foot of snow, which is misleading. You left out the 15% chance of under an inch, and the 50% chance of less than 6". I agree there is always some variability in the models - but the CWG vault, itself, suggests the models were on track despite the very conservative forecast:

"walter-in-fallschurch, I think it's fairly rare to be at this range and see agreement like this for a big event. It could still "go wrong" but it would have to go way wrong for us to get little snow.

Posted by: Ian-CapitalWeatherGang | December 17, 2009 4:05 PM"

Likewise, having a forecast of 7" to 14" change to a forecast of 16" to 24" in less than 12 hours (and only after the snow starts to fall) was slow on the uptake and playing catch-up to other forecasters.

On the upside, it leaves plenty of room for improvement! And despite sounding like a critic, I do appreciate the effort everyone at CWG puts in - its just fun to keep you accountable.

It's not at all misleading. Ian's post (http://voices.washingtonpost.com/capitalweathergang/2009/12/pm_update_snow_is_on_the_horiz.html) from 6:30 p.m. (which several of us collaborated on) which I quote above, 28 hours before the storm started, said 6-12" was most likely. The link and the text is there for everyone to see what we were calling for. Yes- we said there were other possibilities...because it helps people to know what the uncertainty is. You think we overplay the uncertainty and prefer a more deterministic approach. I got that.

In terms of playing catch-up on accumulation totals, I think WUSA was the first to "go big" on accumulations for that particular storm - but otherwise, I recall that we were at least as aggressive as the other media outlets.

Jason's "on the money"-sorry manatt, I was flying into DCA on 12/20 and was glued to CWG for the most reliable forecasts; made it in OK,w/long, slow trek to Franconia via Metro & cab. Best weather predictions in the nation. Back again 12/21/10 & hope DC is spared this year.

A 30% chance was not the "most likely" outcome - you had a 50% chance of less than 6" and a 50% chance of more than 6". At that point the forecast was a coin flip. Saying there was a "potential" for 6" or that 12" was "possible" doesn't change the fact that your forecast spread nearly EVEN odds for everything from less than an inch to two feet. I wouldn't call that "sounding the alarm" - more like "flipping the waffle."